1. Field of the Invention
The present invention relates to aquatic propulsion systems and more specifically to a generally submerged propulsion system for a surface ship.
2. Problems In The Art
Underwater vehicles, such as submarines, are currently pushed through the water using propeller-based propulsion system typically located at the stern of the vehicle. Drag forces acting on the vehicle cause the water in front of, and around the vehicle, to become displaced and turbulent in nature. These drag forces lead to decreased efficiency and a lower overall thrust. Further, such propeller-based propulsion systems cause an increase in the submarine""s noise with an associated increase in speed. This may aid others in detecting the submarine, enabling its destruction.
The increase in noise is due largely to cavitation. Cavitation is the formation of water vapor bubbles caused by rapid propeller movement that creates a vacuum-like area in the incompressible water. The vapor pressure of the water forms a bubble. Surrounding water pressure soon violently collapses the bubble creating substantial noise.
As the speed of the submarine increases, a geometrically increasing wave generated by frontal water resistance limits the increase in speed and contributes to increased cavitation. This wave is the main resistance to high speed travel in surface vessels and plays a role in submarine speed, albeit less when the submarine is at a depth of greater than three submarine diameters below the surface.
In addition, as submarine speed increases, surface friction from turbulence-related viscous shear stress creates a boundary layer of retarded fluid along the surface of the moving vessel. In this boundary layer, eddies of high-speed fluid contact the surface, causing deceleration, sapping the watercraft""s momentum. This boundary layer turbulence increases in magnitude as flow progresses rearward from the bow. Thus nearly all of the vehicle""s surface boundary layer is turbulent. The friction or drag of a turbulent boundary layer is seven to ten times that of a laminar boundary layer, so the possibility of achieving significant reductions in vehicle drag by boundary layer management is attractive.
There have been attempts to reduce boundary layer friction on submarines. For example, in U.S. Pat. No. 4,346,662 to Rogers, a twin hull design incorporates extensive slots in the outer hull. These are kept open by a back-flush pump in the bow at low speed. At high speeds, the bow pump is inactivated while the main pump at the stern exhausts water that has been pulled through the slots by suction. This is the sub""s main propulsion. However, high Reynolds numbers (friction) limited practical application.
U.S. Pat. No. 3,779,199 to Mayer also discusses boundary layer control. Mayer did not solve the boundary layer problem beyond the bow, where the problem mainly exists. The Mayer patent eliminated the conventional propeller and rudder. However, such an arrangement loses efficiency due to poor management of the slipwater at the stern. The small intake diameter at the bow end did not approach the submarine diameter. The water exited through a large number of exit ports behind the intake in the bow region. The large numbers of parallel bow exit slots were arranged in a number of concentric rings. These were used for directional flow and as the only source of propulsion. Due to aforementioned reasons, the Mayer patent was not utilized in a practical submarine application.
Short of eliminating surface resistance and wave generation by other means, stealth submarine speed is slow, and top speed is below that of important surface ships, such as an aircraft carrier. Submarine surfaces are presently coated with rubber to make it less reflective to sonar and mute submarine noise. Stealth is the priority with submarines, it has always been the priority.
Surface water vehicles typically rely on a propeller fan, either by an inboard or outboard stern positioned engine, for propulsion. This application of power to the rear end creates an inverted pendulum, with stability problems. Further, a stern drive arrangement requires the vehicle to be pushed through the water, causing geometrically disproportionate wave drag with any increase in speed. This wave arises from displacing all the water in front of the vehicle, to areas around and behind the vehicle, limiting stability, efficiency and speed. Currently, improved water jet engines are placed at the stem of craft that exhaust the water jet outward, above the waterline. However, they are still subject to the preceding limitations of stem-drive only. They are a variation of the original water-jet engine that accelerates water through a curved passageway. Attempts have been made to address these problems for surface water vehicles. For example, U.S. Pat. No. 5,634,419 to Cymara discloses what is called a xe2x80x9cfront-drive boatxe2x80x9d wherein a propeller propulsion system is located towards the front (bow) of the boat, which is claimed to increase stability of the boat. It corrects the problem of power to an inverted pendulum.
Further, U.S. Pat. No. 4,680,017 to Eller, entitled xe2x80x9cMotorboat Propeller Guard For Improved Performancexe2x80x9d, places a propeller inside a housing with grids configured to attempt to direct propelled water rearward for improved performance (higher speed). It is a jet-like stem drive system. Similar designs, including the stem pumpjet used on modem stealth submarines, follow earlier torpedo pumpjet designs. Cavitation (generation of noisy water vapor bubbles) was reduced in the pumpjet through pressurizing the propeller blade areas and eliminating the propeller tip vortices, making higher speeds at stealth possible. Herein incorporated by reference, U.S. Pat. Nos. 5,383,801 to Chas, U.S. Pat. No. 4,902,254 to Chas, and U.S. Pat. No. 4,831,297 to Taylor et al., disclose propulsion systems for over the water craft that adopt jet engine principles to attempt to increase propulsion. Another propeller based propulsion system is U.S. Pat. No. 5,252,875 to Veronesi, et al, herein incorporated by reference.
Many of the above patents resemble a jet engine in appearance and further resemble a jet engine in the manner in which they are attached to a vehicle; i.e. they hang down from the craft. However, they still have to push a boat through the water, causing wave displacement that increases geometrically with speed, none have twin jet accelerators in sequence and none of the above referenced patents are capable of supercavitation. Currently, only munitions, including rocket powered blunt-nosed torpedoes and high-velocity blunt nosed supercavitating bullets, are capable of any sustained supercavitation. However, the rocket or explosive style propulsion systems have limited range and would be very dangerous for use in passenger travel. There is another patent that deals with decreasing drag to improve stealth and speed. It involves two or more propulsion units in sequence (stages)to power a submarine. A preferred embodiment discusses a two-stage bow propulsion system combined with a conventional stem propeller. The bow stage one vanes and stage two vanes can be selectively employed, individually, or in combination. The two bow stages allow a choice between supercavitation (very fast and noisy), or higher speed stealth (faster silent running) propulsion.
Engaging both twin bow stages on the jet-drive submarine and/or torpedo in U.S. Pat. No. 6,427,618 B1 by Hilleman offers a supercavitating generation platform. Water is incompressible; it is a high-pressure to high-velocity device. The first bow stage minimizes cavitation while it propels the submarine through the water and feeds the second stage. The bow second-stage propeller""s high velocity, combined with the enclosing shroud, the shroud""s trailing edge, high-speed lower-pressure nozzle emission, and water vapor pressure, can create a large long vapor cavity (supercavity). The radial velocity of the tangential-to-flow movement of the stage two vanes can easily achieve speeds of 50 meters per second, to initiate the process of supercavitation, without the need for the submarine itself to reach this speed. This would allow the submarine to smoothly transition to high speed. Supercavity formation around the hull would eliminate surface drag, by placing the hull in a water vapor vacuum cavity.
Wave generated drag is also eliminated, by using the displaced water that causes a frontal wave, to generate the supercavity. Water has to be accelerated to move any craft; there will be nozzle friction loss from resistance, regardless of nozzle location on the craft. Using the bow water that has to be displaced (to allow forward movement), to flow through the propulsor, places the energy of propulsion in an effective location. The frontal wave is incorporated into the resistance of the propulsion system, resulting in less total resistance. As long as the flow of water into the intake is greater than the submarine speed, there will be very little frontal wave generation. The result of less resistance is more speed for a given mass, using the same input energy.
When the supercavitating vanes are deactivated, the nearly straight through flow around the low pitch stationary stage two vanes offer little resistance as the water moves from the stage one propeller to the jet nozzle region. Speed of stealth is increased at speeds below the initiation of cavitation, due to the elimination of frontal wave resistance and also by directing the jet to slightly diverge away from the submarine surface, (to reduce boundary layer problems).
There is also a reduced potential for cavitation in the stage one bow propeller. The stage one blades are increasingly pressurized on their front face as forward speed is moderately increased; however, care must be taken so that increased blade speed does not also increase blade cavitation overall. The blade pressure increase with moderate increased speed retards cavitation on the front face of the stage one blades. As previously discussed, the rate of intake will affect the amount of pressure on the stage one blade faces; i.e. the more the frontal wave is reduced by moving it through the propulsor, the lower the pressure on the frontal blade face will be. Restriction to flow from nozzle friction also suppresses cavitation by pressurizing the back face and tips of the bow propulsor stage one vanes.
This is in sharp contrast to the cavitation seen on conventional submarine stem propulsion, where the suction magnitude in the turbulent stem slipwater increases with speed, which in turn, further increases the potential for cavitation at lower blade speeds. In this embodiment the stem propulsion was nevertheless retained to manage the slipwater suction turbulence and assist propulsion.
Unfortunately, a present state-of-the-art nuclear submarine costs billions of dollars and it takes around ten years to plan and build. Adapting the supercavitating design may even be more costly and time-consuming, due to the major design changes. In addition, the submarine may need to run near the surface to ventilate the supercavity, so as to enhance the cavity""s stability.
Reduction of frontal wave drag and boundary layer drag with surface ships can be attained by raising the surface ship above the water. The speed of surface vessels is improved by the use of hydrofoils; however, speeds, although increased, are limited due to major instability problems and weight considerations. U.S. Pat. No. 5,813,358 to Roccotelli, entitled xe2x80x9cSurface-Piercing Surface Effect Marine Craft,xe2x80x9d uses aerodynamic lift to support the weight of the craft (flying wing), and reduces the immersed parts to a bare minimum in an effort to achieve propulsion and attitude control. U.S. Pat. No. 6,058,872 by Latorre, tries to accomplish the same, using a Catamaran, combining both aerodynamic and hydrodynamic lift.
U.S. Pat. Nos. 5,601,047 and 5,551,369 to Shen shows a supercavitating hydrofoil, which also works at subcavitating speeds. Very high speeds should be attainable by craft driven to supercavitation velocity, as long as the craft remains aloft, supported by the dual-cavitating hydrofoils, and the propulsion does not cause instability (by being above the water). This can be a problem in rough seas. When a hydrofoil is foil-borne, the foils carry 100% of the displacement of the foil craft. If the flow of water over one or more of the foils is interrupted by sea conditions, or flow is detached from stalling or ventilation, the entire hydrofoil is at risk of crashing.
Instability at high speed is also a problem with U.S. Pat. No. 5,359,958 by Guild in the gas-turbine powered xe2x80x9cHigh Speed Watercraft.xe2x80x9d This ocean racer is a hydroplane and it almost approaches supercavitation speed. However, it can be very unstable in turns and in rough seas. Gornstein discusses dual propulsion and hydrofoils in U.S. Pat. No. 4,962,718. As the boat transitions from a water-supported hull, powered by a propeller, to a foil supported craft, an air propeller assumes the task of propulsion. Above-the-water propulsion is unstable.
All present high-speed watercraft are top-heavy and very unstable at high speed. Stability is inversely related to speed. Safety is the major concern. Some time ago the Navy halted all high-speed applications for this reason. They are now re-examining the possibility of deploying troops and tonnage rapidly over water, as the need exists. There is therefore a need for the propulsor to be deeper in the water, providing power in the area of greater resistance, thereby increasing stability and safety.
U.S. Pat. No. 4,981,099 by Holder recognizes the advantage of submerging the propulsion system underwater, i.e. to eliminate bulk necessary for hydrofoil support above water. Four hundred tons is near the practical limit for hydrofoil support. Tonnage increases cubically with increased dimensions, while lifting force of the hydrofoil increases squarely with increased dimensions. U.S. Pat. Nos. 5,503,100 and 4,819,576 by Shaw discuss a hybrid water vessel that comprises a submarine, hydrofoil, and surface ship. One embodiment even discusses a propeller on the front of a submarine, but it is not the twin jet drive of U.S. Pat. No. 6,427,618 by Hilleman, herein incorporated by reference.
Even a reduction of water drag is an advantage. Barbazash, as well as in U.S. Pat. Nos. 5,794,558 and 5,645,008 by Loui, discusses this concept in U.S. Pat. No. 5,355,827. Here improved hydrofoil design supports 70percent of the ship""s weight amidship. It does not lift the surface craft out of the water; however it does raise it somewhat, reducing water displacement and wave drag.
The hydrofoil stability problem has recently been greatly overcome by constantly ventilating the wave-piercing or surface-skimming hydrofoil in U.S. Pat. No. 6,095,076 to Nesbitt. However, small vessel size restriction remains a limitation. Although a great improvement, this fore and aft improved hydrofoil-supported craft is still top-heavy, as substantially all of the weight of the craft is above the water at speed. This is still potentially unstable at high speed and does not lead to good seakeeping in troubled seas. That is unacceptable in a warship; it is even risky in a high-speed ferry.
Therefore, although attempts have been made to increase the speed of surface vessels by the use of hydrofoils and hydroplanes, both size and stability are a problem. Submerging the power plant was a partial solution to the problem, but even though the submarine profile has less water drag than a surface vessel, neither the higher speed provided by supercavitation, nor higher speed at stealth was possible.
Present-day supercavitating propellers are designed for forward speed and are considered incapable of generating a supercavity of any size or stability. However, U.S. Pat. No. 4,681,508 by Kim deals with propeller design to create streamlined supercavitation flow in a centrifugal pump. It generates high suction pressure and has powerful gas and vapor expulsion abilities. It is not only free from cavitation erosion, but also free from the abrasion, damage, or destruction caused by solid matters or gases in the lifting or driving fluid. This design is capable of large supercavity generation and accomplishes this without extreme high speed of rotation or great expenditure of power. The problem with it is that it cannot provide workable forward propulsion in a watercraft. It could be integrated into the twin-jet bow propulsion system discussed previously. The ability to manage some incoming gas makes it particularly attractive.
There is therefore a need to incorporate an unmanned submarine supercavitation propulsion system with a hydrofoil-supported surface craft. In this case, the frontal wave drag and surface friction drag would be minimized, and the resulting stability from a propulsor located deeper in the water could provide improved safety and seakeeping at higher speeds during unfavorable conditions.
In addition, the top speed of present-day stealth surface ships is very low; this limits their applications. Any increase in speed at stealth would be advantageous. Present-day SWATH (small waterplane area twin hull) stealth ships are designed to minimize wake signature. Quiet running on a hydrofoil at stealth speed leaves even less wake than a present-day SWATH stealth ship. Ship profile above water is detected by radar. Submerging the propulsion system reduces the profile supported by the hydrofoil above the water, and can contribute to quieter running. A selective supercavitation option could adapt a sprint-and stealth pattern used by modern submarines. There is therefore a need for a submarine-powered propulsion system which can increase speeds at stealth on a surface ship.
A general feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water which overcomes problems found in the prior art.
Another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water wherein a reduction of drag is caused by decreased surface turbulence along the length of the craft.
A further feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water wherein the reduction of drag is caused by decreased wave generation.
Another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water that provides higher speed at stealth.
A still further feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water that is faster than existing designs, capable of generating a supercavity and traveling in it.
A further feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water that is more efficient at high speed, saving fuel by lowering drag.
A still further feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water which uses one or more propellers to form a jet drive on the bow of an unmanned submersible propulsion system, in combination with a stem propeller or pumpjet.
Another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water wherein a submersed propulsion system supports a surface craft on streamlined struts.
Another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water including a propulsion system which may be maintained while underwater, to raise a surface craft above the water, using hydrofoils.
Yet another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water that combines an underwater mid-craft foil with a wave-piercing or surface-skimming hydrofoil, in order to increase the size and stability of the craft.
Still another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water which is more stable, at all speeds, yet very maneuverable.
Another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water while providing greater safety for the crew through the utilization of twin submarine fore and aft propulsor redundancy, along with a still further surface propulsion option.
An additional feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water that demonstrates a resistance to sinking with a compromised hull, compromised superstructure, or compromised propulsion system.
Yet another feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water with the capability for increased stealth through completely submerging and maneuvering the surface vessel portion below the waterline.
As still further feature of the present invention is the provision of a method and apparatus for propelling a surface ship through water including a safer location for fuel storage.
These, as well as other features and advantages of the present invention, will become apparent from the following specifications and claims.
The present invention generally comprises a propulsion system for a submarine-powered surface vehicle. The submarine propulsion system generally comprises at least two propellers, mounted on a hub, the foremost located in a shroud on the bow end, which forces water into an inlet and out an outlet through a nozzle, thereby increasing the waters velocity and thereby producing a propelling force. It is a plug-type nozzle jet, formed by the shroud and body of the submarine. The jet exhaust is slightly diverged away from the exterior of the submarine. This divergence minimizes boundary layer friction drag and creates a counter-current turbulence in a positive direction. This jet propelling force can reduce frontal drag to increase speed at stealth, or generate a supercavity, permitting nearly drag-free very high speeds. Surface modification may also decrease surface drag (below the speed of cavitation), to complement the reduced frontal drag, thereby increasing the speed of stealth.
The unmanned submarine is attached, via streamlined struts, to a surface craft, engineered primarily for stealth and superior seakeeping. A SWATH (small-waterplane area twin-hull) craft has two submarines, one under each hull, connected by streamlined struts. A submerged, mid-craft hydrofoil supports the majority of the surface craft""s weight while underway. It connects the twin submarine propulsion systems below the SWATH vessel. Wave-piercing or surface-skimming fore and aft hydrofoils, contribute support to a minor portion of the surface craft""s weight while underway, providing surface stability at speed. Because the wave-piercing (surface-skimming) hydrofoils are continually ventilated by the atmosphere, there is a unique cavitation and stability advantage with the combined use of two types of foils at stealth speed. The two types of hydrofoils support the surface craft above the water on streamlined struts at speed; however, the submarines and mid-craft foil remain submerged, adding significant underwater mass and stability at speed. When the surface craft is in bad seas or cruising at slow speed, the stable, small waterplane hull configuration supports the craft lower in the water. The submarine(s), along with the mid-craft foil, act as a keel, minimizing both pitch and roll, to further enhance good seakeeping. Having the propulsion system lower in the water is more stable at all speeds, than any location on or above the surface; it places the source of propulsion farther into the area of greatest resistance.